cronjob for creating two files with 2 GB of size each

Question

I need to create a cron job that runs every hour.

This cron job should create two files, each 2GB, one should be created from /dev/urandom and the other one from /dev/zero and should be written to a specific filesystem/directory.

How would I do this?

Answer 1

You can create these files with dd command like below:

dd if=/dev/zero of=/tmp/ZeroFile count=10000 bs=200000

dd if=/dev/urandom of=/tmp/UrandomFile count=10000 bs=200000

and for scheduling this jobs with cron you have to put this command in your cronjob:

1. crontab -e
2. write it: 0 */2 * * * /path/to/your/command

Answer 2

Edit the user crontab

crontab -e

Then add the following lines

0 * * * * dd if=/dev/random of=/tmp/myfile1.dat bs=$(( 1024 * 1024 * 1024 )) count=2
0 * * * * dd if=/dev/zero   of=/tmp/myfile2.dat bs=$(( 1024 * 1024 * 1024 )) count=2

Or edit /etc/crontab and substitute user_to_run_as with the user you want to run the command as

0 * * * * user_to_run_as dd if=/dev/random of=/tmp/myfile1.dat bs=$(( 1024 * 1024 * 1024 )) count=2
0 * * * * user_to_run_as dd if=/dev/zero   of=/tmp/myfile2.dat bs=$(( 1024 * 1024 * 1024 )) count=2

I tried it out and using /dev/random doesn't give you an exact file size. "Why does dd from /dev/random give different file sizes?" seems to give the explanation for this